1. Technical Field
The present invention relates to gas barrier films and electronic devices and, more specifically, to a gas barrier film used in an electronic device generally for an organic electroluminescence (EL) element, a solar cell element, a liquid crystal display element, or the like, and an electronic device in which the gas barrier film is used.
2. Description of Related Arts
Conventionally, gas barrier films formed by laminating a plurality of layers containing thin films of metal oxides such as aluminum oxide, magnesium oxide, and silicon oxide on plastic substrate and film surfaces have been widely used for packaging articles needing to be shielded against various gases such as moisture vapor and oxygen, e.g., food products, industrial products, pharmaceutical products, and the like.
In addition to packaging applications, development to flexible electronic devices for solar cell elements, organic electroluminescence (EL) elements, liquid crystal display elements, and the like, having flexibility has been demanded and many examinations thereof have been made. However, these flexible electronic devices require very high gas barrier properties equivalent to a glass base level. Any gas barrier film having such a high barrier property has not yet been currently obtained.
As a method for forming a gas barrier film, there has been known a gas phase method such as a chemical deposition method (plasma CVD method: Chemical Vapor Deposition) by which a film is formed on a substrate while oxidizing an organosilicon compound represented by tetraethoxysilane (TEOS) by oxygen plasma under reduced pressure or a physical deposition method (vacuum deposition method or sputtering method) by which metallic silicon is evaporated to be deposited on a substrate using a semiconductor laser in the presence of oxygen.
Inorganic film formation methods by these gas phase methods have been preferably applied to formation of inorganic films including silicon oxide or silicon oxynitride since a thin film with accurate composition can be formed on a substrate. However, it is very difficult to form an inorganic film without any defect and a gas barrier film obtained by an inorganic film formation method does not industrially sufficiently have a gas barrier property required by a flexible electronic device. Although such an examination that a film thickness is simply increased to laminate a plurality of inorganic films by a gas phase method has been made, any film having a desired gas barrier property has not been obtained since a defect continuously grows and the number of cracks is contrarily increased.
In contrast, there has been also examined a gas barrier film obtained by forming a laminate in which inorganic films and organic films are alternately laminated more than once by a gas phase method. According to the gas barrier film, the film thickness of the inorganic films can be increased without continuously growing any defect and a gas barrier property can be further improved utilizing a so-called maze effect that a gas transmission path length is increased due to the different in-plane direction positions of the defects of the inorganic films. However, it cannot be said that a sufficient gas barrier property can be currently realized even in the gas barrier film in which such organic films are combined. It is considered that practical use thereof is also difficult in view of a cost in consideration of a complicated production step, significantly low productivity for performance, and the like.
As one of methods for solving the above-described problems, there has been examined a method by which a solution of an inorganic precursor compound is coated on an inorganic film formed by a gas phase method to make a multilayered constitution to improve a gas barrier property.
For example, Patent Literature 1 discloses a gas barrier film in which a solution containing polysilazane as an inorganic precursor compound is laminated and coated more than once on a gas barrier layer formed on a resin base by a vacuum plasma CVD method and then subjected to heat-treating to form a silicon oxide layer and thereby a gas barrier property is improved. It is described that the mechanism of improving the gas barrier property is not clear but examples of the factors of the improvement in gas barrier property include the good adhesiveness of a lamination interface and the functioning of a polysilazane heat treatment layer as a protective layer for a gas barrier layer. However, since this gas barrier film requires heating at 160° C. for 1 hour in the heat treatment of the polysilazane coating layer, it is significantly poor in productivity. Further, since the improvement level of a moisture vapor transmission rate by applying the polysilazane heat treatment layer is around 1/10 and the arrival level of the moisture vapor transmission rate is around 5×10−2 g/m2·day, the gas barrier property has been far inferior to that demanded by a flexible electronic device.
Patent Literature 2 disclosed an organic EL apparatus in which a gas barrier layer formed on an organic EL element by a plasma CVD method is coated with polysilazane, followed by carrying out baking treatment to form a protective layer. It is described that water can be prevented from infiltrating by covering the defect of the gas barrier layer that can be formed and generated by the plasma CVD method, with the protective layer formed by baking treatment of polysilazane. It is further described that water can be prevented from infiltrating by making polysilazane in a semidry state to make an unreacted part remain and by reacting the part with infiltrating water. However, the protective layer mainly containing silicon oxide, formed by the baking treatment of polysilazane, does not substantially have such a gas barrier property as demanded by a flexible electronic device. Further, although the unreacted polysilazane film gradually reacts with water in the atmosphere to becomes silicon oxide, it cannot be said that there is the long-term function of preventing water from infiltrating since the reaction is completed for several days to several months, and unreacted water cannot be prevented from infiltrating since the reaction rate of the reaction is not high.
In contrast, Patent Literature 3 discloses an electronic component in which an inorganic film formed on a resin substrate by a gas phase method and a polysilazane film are sequentially formed. Like Patent Literature 2, it is described that water is prevented from infiltrating into an electronic component element portion by reacting the unreacted part of the polysilazane film with water and water infiltration can be suppressed for a long term by reducing the amount of water infiltrating into the polysilazane layer by the inorganic film formed by the gas phase method. However, as mentioned above, polysilazane heat-cured and moist-heat-cured films in themselves do not substantially have any gas barrier property, a reaction of unreacted polysilazane with water is not rapid, a water transmission prevention effect is therefore insufficient, and, thus, any gas barrier property demanded by a flexible electronic device cannot be obtained.
Patent Literature 4 discloses, as another attempt, a transparent laminate in which a silicon nitride film is formed on a transparent substrate under specific CVD conditions. The transparent laminate described in Patent Literature 4 prevents water from infiltrating by reacting a silicon nitride compound with water. It is described that the silicon nitride film becomes such a chemically unstable film as being able to be oxidized in an atmosphere in which oxygen and water are present by forming the silicon nitride film under the specific CVD conditions and oxygen and moisture vapor can be prevented from passing through the transparent laminate by the absorption of oxygen and moisture vapor by the film. However, since the way in which the CVD-formed silicon nitride film to be originally function as a gas barrier layer become the chemically unstable film is not explained at all and any film composition is not mentioned, there is a concern whether or not the chemically unstable silicon nitride film can be stably formed. Further, as for the transparent laminate described in Patent Literature 4, since it is necessary to maintain the silicon nitride film in a chemically unstable state by forming chemically stable gas barrier films on both sides of the silicon nitride film, at least three inorganic layers formed by a gas phase method such as CVD are substantially necessary and the transparent laminate described in Patent Literature 4 has a constitution with poor productivity.
On the other hand, there has been made the examination of expressing a high gas barrier property by converting polysilazane in itself.
As mentioned above, polysilazane with (Si—N) as a basic structure changes to silicon oxide in the state of comparatively small volume shrinkage since nitrogen is directly substituted with oxygen by oxygen and moisture vapor in the air by heat treatment or moist heat treatment. As a result, a comparatively dense film can be obtained with a few defects in the film due to the volume shrinkage.
However, the formation of the dense silicon oxide film by the thermal conversion of polysilazane requires a high temperature of 450° C. or more and it was not able to be adapted to a flexible substrate such as a plastic.
As means for solving such a problem, there has been proposed a method for forming a silicon oxide film by irradiating a coating film formed by coating a polysilazane solution, with vacuum-ultraviolet light.
Vacuum-ultraviolet light with a wavelength of 100 to 200 nm (hereinafter also referred to as “VUV” or “VUV light”) has a larger light energy than bonding force between respective atoms of polysilazane. The silicon oxide film can be formed at a comparatively low temperature by making an oxidation reaction due to active oxygen or ozone proceed while directly cutting an atomic bond by the action only of a photon by using the light energy (light quantum process).
Patent Literature 5 and Non Patent Literature 1 disclose a method for producing a gas barrier film by irradiating a polysilazane compound coating film with VUV light by using an excimer lamp. However, production conditions are not examined in detail and the gas barrier property of the obtained gas barrier film is far inferior to the gas barrier property demanded by a flexible electronic device. Furthermore, the relationship between the film composition of the polysilazane converted film obtained by irradiating the polysilazane compound with the VUV light and the gas barrier property has been currently hardly examined.
As described above, there has been demanded a gas barrier film enabling compatibility between a very high barrier property required by a flexible electronic device or the like and productivity.